1. Field of the Invention
The present invention relates to an optical access network system whereby a provider and a subscriber communicate by means of a code division multiplexing (CDM) system in a passive optical network (PON).
2. Description of Related Art
Optical access network systems that are constituted by connecting a provider (also called a ‘center’ hereinafter) and a plurality of subscribers (also called ‘users’ hereinafter) via a PON have attracted attention. In the following description, a provider-side device is also referred to as the optical line terminal (OLT) and a subscriber-side device is also known as the optical network unit (ONU).
A PON is a network in which a single optical fiber channel is split into a plurality of optical fiber channels by connecting a star coupler which is a passive element midway along the optical fiber channel and a plurality of optical terminal devices are connected in a star shape with the star coupler at the center thereof. By adopting a PON for a network that joins a center and its users, an optical fiber channel between the center and the star coupler can be shared by a plurality of users, whereby equipment costs can be suppressed.
For efficient usage of communication resources in a PON system, a best effort service is the norm. However, in order to simultaneously execute speech (call) services, video services, data communication services and so forth as the application services which are provided in the PON system, it is necessary to secure a bandwidth with minimal restrictions for speech and video communications and so forth. Furthermore, in a PON network that is constructed in an enterprise, it is necessary to update a database and temporarily large-capacity communications are required. Thus, in order to always guarantee efficient communications in correspondence with the status in which the PON network is used, rational bandwidth control and management of the bandwidth used are sometimes required.
Therefore, a time slot allocation control device that permits efficient bandwidth control as a result of the optical line terminal allocating time slots to the respective optical network units in accordance with the standby information amount during the operation of each of the optical network units or the increase in the standby information amount has been disclosed (See Japanese Application Laid Open No. H10-107755, for example). With the time slot allocation control device, the optical line terminal first issues a request to all the optical network units to send individual management slots in one or more time slots while the respective optical network units report the standby information amount of the optical network units to the optical line terminal at the point where the transmission of the individual management slots is requested by the optical line terminal.
The optical line terminal allocates the time slot for the next frame based on the standby information amount reported by the respective optical network units. The capacity allocated to each of the optical network units is determined as being proportional to the standby information amount, for example. Alternatively, the capacity allocated to each of the optical network units is determined using a method such as that of performing correction to allocate the lowest capacity, allocating the excess in the allocated value, and uniform distribution by the terminals to which a capacity equal to or more than the lowest capacity has been allocated.
The above device affords simple control and also billing processing in accordance with the capacity allocated to each of the optical network units.
In addition, a dynamic time slot allocation system that performs time slot allocation efficiently in accordance with changes in the transmission amount has been disclosed (See Japanese Patent publication No. 3490583, for example). This dynamic time slot allocation system is a system in which a plurality of optical network units and one optical line terminal are connected by a PON. A poling system is adopted for this system. The poling system is a system in which the optical line terminal does not make sequential transmission requests to the optical network units or it is hard for the optical line terminal to make such transmission requests. The optical line terminal is able to transmit data to the respective optical network units at any time but the respective optical network units are able to transmit data to the optical line terminal only given confirmation of a request to do so.
That is, in a dynamic time slot allocation system, the optical network units comprise means for issuing a poling request to the optical line terminal and the optical line terminal comprises means for identifying the poling requests sent by the optical network units and allocating a time slot in accordance with the poling standby information amount to a spare domain of lowest guarantee poling. As a result, the whole system, in which a plurality of optical network units and one optical line terminal device are connected via a PON, is constituted to allow the allocation of time slots to be performed efficiently and dynamically.
As mentioned earlier, this dynamic time slot allocation system is a system that is capable of guaranteeing each optical network unit the lowest bandwidth and of controlling the bandwidth dynamically by opening the required bandwidth for the respective optical network units on the basis of information on the spare bandwidth and so forth. That is, this system can be said to be a system that is devised in order to be able to efficiently utilize the communication resources.
In order to be able to adapt to packets of a variety of packet lengths, the time slot allocation control device must perform control to separate or combine packets. Further, a division signal for the separation of packets must be subjected to delay control. However, it is difficult for the time slot allocation control device to be able to execute control in order to perform this separation or synthesis and to execute division signal delay control.
Furthermore, with respect to the method executed by the dynamic time slot allocation system, a situation where it is not possible to adapt to a case where another user temporarily requires a large capacity bandwidth in the circumstance that a specified user always uses a large capacity bandwidth arises.